The two acellular O2 carriers currently being evaluated as red cell substitutes are hemoglobin solutions (SFH) and fluorocarbon emulsions (FC). At the present time, clinical testing has been limited to the FC. Preliminary data from the patients we have studied have suggested an inadequate rate of red cell rcovery compared with the plasma disappearance of the FC. There is little data concerning the normal red cell recovery in severe acute normovolemic anemia in humans. It is therefore difficult to assess the influence of SFH or FC in this setting. We intend to address these issues using the baboon as the animal model. The principal goals of this proposal are: 1) To establish a normal red cell regeneration time in moderate and severe anemia. 2) To assess the effect of supplemental oxygen alone, Fluosol-DA alone and oxygen and Fluosol on the normal red cell regeneration time. 3) To assess the effect of tetrameric and polymerized hemoglobin solutions on the normal red cell recovery time. The results of these studies will provide information about the normal red cell recovery time in acute anemia, and the influence of both acellular O2 carriers in this setting.